Isn't that a bad attitude though. Mono isn't obsolete and I don't think it'll ever be. PA systems use mono for the most part, and I'm sure there are people out there who still listen to mono. In my opinion, as a music artist you should want your music to sound good no matter how it's played. And if it sounds good in mono, it'll sound good(and in some cases, super good) in stereo, but the reverse isn't the same.
I can honestly say since switching to mono while mixing, I've learned about how elements interact with each other when they're sitting EXACTLY on top of each other. Once you get them working together in mono, then editing them in stereo will yield a more crisp result with your work and more room for your music to "breathe" I guess you could say.
But of course everyone is different and some people can get away just fine with mixing solely in stereo. I for one am glad I switched over. I haven't produced anything yet with this, but when I do, there will be a noticeable difference in my previous work xD.